Interview Q&A / SQL & DBMS

1. What is a covering index?

   A covering index contains every column a query needs, so the database answers from the index alone without visiting the table. This is called an index-only scan and avoids extra lookups.

   IntermediateQuick answer1 min read
2. What is denormalization and when would you use it?

   Denormalization deliberately adds redundant data, such as copied columns or precomputed totals, to avoid joins and speed up reads. It trades storage and write complexity for read performance, and suits read-heavy reporting.

   IntermediateQuick answer1 min read
3. Why might the database ignore an index on a column?

   Common reasons are wrapping the column in a function, a leading wildcard in LIKE, a type mismatch, a condition that matches most rows, or outdated statistics. The optimizer then prefers a full scan.

   IntermediateQuick answer1 min read